Prior art airflow meters comprise film resistors which function not only as electric heaters, but also as temperature-detecting means for ascertaining temperature of heated air. Some prior art air flow meters comprise temperature-dependant resistors for detecting the temperature of non-heated air. Current flowing through film resistors is regulated to provide a constant differential in temperature between a film resistor and a temperature dependant resistor. By measuring voltage applied to the film resistor, the mass flow rate of air can be determined as the voltage required is dependent on the mass flow rate of air. In this type of aiflow meter, where a temperature dependant resistor is not provided and the current of the heater resistor is regulated to provide a constant temperature of the film resistor, the voltage applied to the film resistor is proportional to the volume flow of air and can be detected and converted to provide the desired information.
Generally, hot element anemometers comprise at least one filament of conductive metal, commonly known as a detector filament, disposed in one arm of a Wheatstone bridge circuit and supplied with a source of electrical energy. The detector filament is heated by electric current, while immersed in the fluid flow stream. The passage of the fluid stream over the hot filament cools the filament and consequently causes its electrical resistance to vary. A difference of potential, the value of which is proportional to the speed of the fluid flow, appears across the measuring diagonal of a Wheatstone bridge. In other known electrothermal air mass sensors, a resistance layer applied to a flat support is traversed by a current resulting in the generation of heat. Depending on the velocity of flow in the air surrounding the sensor, a greater or lesser amount of heat is removed from the resistance layer and the support. By the use of a resistant material having a positive or negative temperature coefficient, the flow can be controlled such that a constant temperature difference above the temperature of the air is maintained. The current required to maintain is this difference a measure of the velocity of flow.
The most popular prior art devices rely on either constant temperature (resistance) or constant current. Using these prior art devices requires considerable knowledge, skill and expertise to obtain consistent and accurate measurements. The operator must first establish the resistance of the sensor at ambient temperature. The adjacent arm of the Wheatstone bridge must then be adjusted to a multiple of the sensor resistance. Next, the offset voltage must be adjusted to provide the necessary current through the sensor. And finally, an inductor must be adjusted, using a square wave input, for proper speed response.
All of these adjustments are necessary because of the Wheatstone bridge network commonly used in prior art devices. An anemometer may be operated with a constant temperature or a constant current to obtain changes in the instantaneous power dissipated as a measure of heat transfer which in turn is a measure of fluid velocity. In a constant temperature operation, the Wheatstone bridge performs a significant role with conflicting requirements. While it is necessary to have an exact balance for true constant temperature operation, such a situation leads to instability due to infinitely large gains being required. In practice the system frequency response and stability are optimized by adjusting the balance reactive element and the offset voltage current.